1. Field of the Invention
This invention relates to a conductive roller, and a process cartridge and an electrophotographic apparatus which have the conductive roller.
2. Related Background Art
A number of methods are conventionally known as methods for electrophotography. Copies are commonly obtained by forming an electrostatic latent image on an electrophotographic photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner to form a visible image as a toner image, transferring the toner image to a transfer medium such as paper, and thereafter fixing the toner image to the transfer medium by heat and/or pressure. Also, toner particles not transferred to the transfer medium and remaining on the photosensitive member are removed through a cleaning step from the surface of the photosensitive member.
As charging assemblies for electrophotography, corona charging assemblies have conventionally been used. In recent years, contact charging assemblies have been put into practical use in place of the former. This aims at low ozone and low power consumption. In particular, a roller charging system making use of a conductive roller as a charging member is preferably used in view of the stability of charging.
In the roller charging, a conductive elastic roller is brought into pressure contact with a member to be charged (charging object) and a voltage is applied thereto to charge the charging object electrostatically. Stated specifically, the charging is performed by causing electric discharge from the charging member to the charging object, and hence the charging takes place upon application of a voltage above a certain threshold voltage. For example, where a charging roller is brought into pressure contact with an organic electrophotographic photosensitive member having a thickness of 25 μm (an OPC electrophotographic photosensitive member), the surface potential of the electrophotographic photosensitive member begins to rise upon application of a voltage of about 640 V or above and, at voltages above that voltage, the electrophotographic photosensitive member surface potential increases linearly at a gradient 1 with respect to the applied voltage. This threshold voltage is hereinafter defined as charging start voltage Vth.
That is, in order to attain an electrophotographic photosensitive member surface potential Vd considered necessary for electrophotography, a DC voltage of Vd+Vth that is higher than is necessary must be applied to the charging roller. Such a method of applying only DC voltage to the contact charging member to perform the charging in this way is called DC charging.
In DC charging, however, it has been difficult to control the potential of the electrophotographic photosensitive member to the desired value, because any environmental variations may cause variations in electrical resistance of the contact charging member and also because any changes in layer thickness due to abrasion of the electrophotographic photosensitive member surface may cause variations in the Vth.
Accordingly, in order to achieve more uniform charging, an AC charging system is used in which a voltage formed by superimposing an AC component with a 2×Vth or higher peak-to-peak voltage, on a DC component corresponding to the desired Vd is applied to the contact charging member. This is a system aiming at a potential-leveling effect which is attributable to AC, where the potential of the charging object converges to the Vd, the middle of the peak of AC voltage, and may hardly be affected by external disturbance such as environmental variations. In recent years, such contact charging in which the roller-shaped contact charging member is brought into contact with an image bearing member (photosensitive member) to charge the surface of the image bearing member electrostatically is in wide use. The contact charging member has advantages such that it has simple structure and ozone is generated only in a very small quantity.
An example is shown in which a conductive base layer is covered thereon with a conductive seamless tube to form a surface layer, and a multi-layer tube is disclosed which is formed of layers having different conductivities. As a method for manufacturing such a charging member, a method is available in which in the above conventional techniques the surface layer is formed by insertion (see, e.g., Japanese Patent Application Laid-open No. H05-096648).
However, a charging roller obtained by such a method makes use of a thermoplastic resin in the surface layer, and hence, because of a low cross-link density of the resin, any of toner, external additives and photosensitive-member abrasion dust may adhere to the roller surface during the image formation to cause faulty images in some cases.
As a means for solving this problem, a method is commonly proposed in which surface roughness (Rz: ten-point average roughness) is made low to smooth the surface (see, e.g., Japanese Patent Application Laid-open No. 2000-137369). However, where a seamless tube formed basically of a thermoplastic elastomer is produced by the method disclosed in Japanese Patent Application Laid-open No. 2000-137369, the tube is soft because of its own properties, and hence it has not been able to be said that the faulty images due to the adhesion of photosensitive-member abrasion dust can sufficiently be remedied only by lowering surface roughness (Rzjis: ten-point average roughness) to make the surface smooth.
As another means for preventing the toner and external additives from adhering to the roller surface, a method is proposed in which a sponge, a brush or a blade is pressed against the charging roller at a suitable pressing force to clean the charging roller, retaining the charging capability thereof (see, e.g., Japanese Patent Application Laid-open No. H06-149020).
With such construction, however, in the case of the blade, a container used exclusively for holding therein the toner removed is required, and this makes it difficult to make the apparatus compact. In the case of the sponge or brush, cleaning ability lowers when the quantity of toner closed in pores or brush hair comes more than a stated quantity, resulting in a low adhesion preventive effect. There have been such problems.
Accordingly, a method is proposed in which a roller is coated on its surface with an inorganic compound of various types to improve the surface properties of the roller so that the toner and external additive can be made to less adhere to the surface (see, e.g., Japanese Patent Application Laid-open No. 2002-031958).